The present invention relates to an expander for a compander system, and more particularly for a compander used for the transmission and reception of multichannel sound signals.
In the United States it has been proposed to transmit a television signal having stereo audio components. In one proposed system, known as the "Zenith" system, an L-R signal representing the difference between left (L) and right (R) stereo signals is transmitted as an amplitude modulated (AM) suppressed carrier at twice the horizontal scanning frequency. To maintain compatibility with existing monophonic (mono) receivers, the monaural signal, which consists of the L+R sum signal, is transmitted as a frequency modulated (FM) carrier in the conventional fashion. In a television receiver, the L+R and L-R signals are demodulated and then matrixed (added) to produce L and R signals. It has also been proposed to transmit an audio signal for a so called "second audio program" (SAP), e.g., for providing the main program in a second language. In the "Zenith" system, a frequency modulated carrier at five times the horizontal frequency is used for the SAP. For a more complete description of the Zenith system see pages 110-111 of "Television Multichannel Sound", Vol IA published by the National Association of Broadcaster, Nov. 9, 1983; and also U.S. Pat. No. 4,339,772.
For both the L-R and SAP signals it is also proposed to use a so called "dbx" companding system in which variable preemphasis (compression) of high frequency components at the transmitter and complementary deemphasis (expansion) at the receiver is used in order to improve the signal-to-noise (S/N) ratio of the corresponding reproduced signals. The "dbx" companding system is described in detail in the publication entitled "Compandor Complexity Analyses" published by the Broadcast Television Systems Committee of the Electronic Industries Assocication (Supplement of Dec. 5, 1983). Basically, the purpose of providing substantial high frequency content in the transmitted signal is that it has been found that the greater the high frequency content of a transmitted signal the less perceptible will be noise (i.e., the more noise will be masked).
A companding system is not used for the L+R signal since, if it were, the ability of existing receivers to properly reproduce monaural signals would be disturbed. Furthermore, the L+R signal is less subject to being contaminated by interference than the L-R or SAP signals.
Unfortunately, due to the presence at the input of the expander of undesired signals having frequencies above the audio band, as will be explained below in detail, the deemphasis in the receiver will not match or be complementary to the preemphasis in the transmitter. Thus, the received audio signal will not be restored to its original level. In addition, the phase shifts caused by the preemphasis and deemphasis circuits will also no longer be complementary. In the case of stereo reproduction, this results in reduced stereo separation since the L-R and L+R signals when matrixed will not combine to produce the proper L and R signals.
It has been proposed to use a filter in the audio signal path of the L-R expander in order to remove the undesired signals. However, this requires the use of a filter in the L+R audio path that matches the filter in the L-R path in amplitude and phase response in order to maintain stereo separation. The use of two filters is expensive, and the requirement that they be matched requires the use of close tolerance filter components, which further increases the cost of the filters.